High-frequency furnace for inductive heating



L. BLOK Feb. 13, 1962 HIGH-FREQUENCY FURNACE FOR INDUCTIVE HEATING Filed Dec. 18, 1959 FIGJ INVENTOR LOURENS BLOK AGEN I 3,021 413 I HIGH-FREQUENCY FURNACE FOR INDUCTIVE HEATING Lourens Blok, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Dec. 18, 1959, Ser. No. 860,424 Claims priority, application Netherlands Dec. 22, 1958 9'Claims. (Cl. 21910.75)

load matching the coil of this tuned circuit is variably coupled with a coupling coil which is connected in the load circuit and is provided with terminals for connection to a heating coil.

The variable coupling between the circuit coil and the coupling coil can be designed in various ways, for example by arranging the circuit coil and the coupling coil so as be-dis'placeable relatively to one another, by the use of a movable ferromagnetic or copper coil, and the like.

In order to obtain maximum power output, in such a high-frequency furnace the oscillator valve is matchedto the load circuit by adjustment of the variable coupling between the circuitcoil and the coupling coil, the load circuit usually having a low resistance and being pro* vided with a heating coil having a small number of turns, so that the inductance of this heating coil lies between,

for example, 0.02 th, and l b. Such high-frequency furnaces can be used with advantage for various applications, in particular for high-frequency heating with very is also provided with terminals for connection of a second 1 heating coil, which terminals are includedin the tuned anode circuit of the oscillator valve in series with the circuit coil, and with-an interconnection device associated with the connecting terminals of the two heating coils so that in one positionof the interconnection device the first heating coil is connected to the'coupling coil and the terminals for connecting the second heating coil to the tuned anode circuit are short-circuited, while in asecond position of the interconnection device the second heating coil is connected in series with the circuit coil of the tuned anode circuit of the oscillator valve, the terminals for connecting the first heating coil to the coupling coil being short-circuited.

The interconnection device can be designed in various manners, for example as interconnecting strips, switches I and the like.

which lattercoil has a larger turns number than the first heating coil, for example, for achieving more even heating of articles of irregular shape or for high-frequency heating through long connecting leads to the heating coil. Thus, the inductance of the heating coil can be'inc'reased, for example, from 1 .h. to 40 h.

In order that the invention may readily be carried out,

3,021,413 Patented Feb. 13, 1962 ICC . 2 embodiments thereof will now be described, by way of example, with reference, to the accompanying diagrammatic drawings, inwhich:

FIG. 1 shows a high-frequency furnace in accordance with the invention,

FIG. 2 shows another embodiment of a high-frequency I furnace in accordance with the invention, while,

FIG. 3 is a furtherdevelopment of the high-frequency furnace of FIGS. 1 and 2. r FIG. 1 shows a high-frequency furnace for inductive heating in accordance with the invention, which has a rated power of, say, 25 kw.

In this furnace, the high-frequency energy required for heating is taken from a thermionic valve oscillator comprising a triode 1. ,The anode circuit of the valve oscillator, which is designedas a Colpitts circuit, contains a resonant circuit 2 which determines the oscillator frequency and comprises a circuit coil 3, 3' shunted by two series-connected circuit capacitors 4 and 4'. One end of the circuit 2 is connected, through a blocking capacitor 5, to the anode and the otherend, through a grid capacitor 6, to the control grid of the triode 1, a leakage resistor 7 being connected, in series witha choke coil 8, to the earthed cathode of the triode 1. The junction point of the capacitors 4, 4 is connected to earth by a connecting lead, while the supply voltage for the anode of the triode 1 is taken through a choke 9 from a direct-voltage source 10. I

The current produced in the resonant circuit 2 by oscillating of the circuit arrangement is used for heating a workpiece 14 arranged in a heating coil 13, and for this purpose the circuit 2 is inductively coupled to a coupling coil 11 having terminals 12 for connection to the heating coil 13. The load circuit is a low-resistance circuit in i which a heating coil 13 is used comprising a single turn and having an inductance of approximately 0.6,uh.

For matching the low-resistance load circuit to the 0s: v

cillator valve under varying operating conditions, the circuit coil 3, 3' and the coupling coil 11 are coupled to one another variably with the use of a copper coil core 15, which can be displaced in the axialdirection of the circuit coil 3, 3 so that the transformation ratio can be varied, for example, from 11:1 to 6:1, Due to transformation of the circuit current a very large heating current of, for example, approximately 1200 amps flows in the load circuit of thehigh-frequency furnace with a resulting very high energy density in the heating coil 13, which may be used to advantage for the progressive hardening of steel shafts.

In order to extend its field of application,the high-frequency furnace described is provided with terminals 17 for connection of a second heating coil 16, which is connected in the resonant circuit 2 in series with the circuit coil 3, 3,'and also with an interconnection device associated with the connecting terminals 12 andl7 of the coupling coil 11 and circuit coil 3, 3, respectively, the first heating coil 13 being connected to a coupling coil 11, and the connecting terminals for the second heating coil 16 being short-circuited in the first position of the connection device, whereas in a'second position of the interconnection device the second heating coil 16 is connected in series with the circuit coil 3, 3' of the circuit 2 of triode 1 and the connecting terminals 12 of coupling coil 11 are short-circuited. The second heating coil 16 has,,for example, 12 turns, its inductance being about 151.011. I

p In the embodiment shown, the interconnection device comprises two switches 18 and 19 and in the first position of the interconnection device the switch 18 connectsthe second position of the interconnection device the switch 19 connects the heating coil 16 in series with the circuit coil 3, 3 and the switch 13 short-circuits the connecting terminals 12 of the coupling coil 11 through an interconnection '21. For simple operation of the interconnection device, the two switches 13 and 19 may be mechanically coupled together.

lln addition to the high-frequency furnace for high-frequency heating through the low-resistance load circuit described hereinbefore, the high-frequency furnace shown has an extended field of application in that it is also suit able for high-frequency heating through a high-resistance load circuit. The high-resistance load circuit of the highfrequency furnace consists of the second heating coil 16, which for this purpose is connected in series with the circuit coil 3, 3' by means of the switch 19, the coupling coil 11 being short-circuited through the interconnection 21.

In order to obtain load matching between the load circuit and the oscillator impedance when using the high resistance load circuit under variable operating conditions, the variable coupling of the circuit coil 3, 3' to the coupling coil 11 is set to a suitable value by means of a copper coil core which is displaceable in the axial direction of the circuit coil, for, when the coupling of the circuit coil 3, 3 to the coupling coil 11, which is short circuited through the interconnection 21, is varied, the inductance of the resonant circuit 2 varies so that the power delivered to the high-resistance load circuit can be adjusted to a maximum value, while in particular the circuit inductance together with the second heating coil 16 forms an auto-transformer, the transformation ratio of which can be varied between 10:6 and 1012.5. Thus, the high-frequency furnace shown can also be used for high-frequency heating by means of a high-resistance load circuit and this is of advantage, for example, for high frequency heating through a long connecting lead to the heating coil 16, for in this manner a decrease in efiiciency owing to the long connecting lead is materially reduced, since the heating current flowing through heating coil 16 of the high-resistance load circuit is a fraction of the heating current in heating coil 13 of the low-resistance load circuit, for example, approximately 200 amps.

Thus, the high-frequency furnace described can be adapted at will to high-frequency heating through a lowresistance load circuit and to high-frequency heating through a high-resistance load circuit, the load matching being controlled in the same manner in both cases by variation of the coupling of the circuit coil to the coupling coil 11. In this manner, the high-frequency furnace combines simplicity in construction and operation with a considerable expansion of the field of application so that it is highly suitable for use in practice.

In order to ensure that, when the high-resistance load circuit is used, the value of the voltage across the second heating coil 16 is kept to a minimum with'respect to earth, the circuit coil is divided in two parts 3, 3 and the terminals 17 of this coil are arranged between the two coil parts 3 and 3', while these coil parts 3 and 3' are in the same ratio to one another as the reactances of the circuit capacitors 4- and 4', so that always some point of the second heating coil 16 is at earth potential. In the embodiment shown, the circuit capacitors 4 and 4, and hence the coil parts 3 and 3' also are equal to one another, while furthermore, in order to adjust the grid excitation to its optimum value, there is connected in parallel with the series combination of the grid resistance 7 and the choke 8 a capacitor 22, which together with the grid capacitor 6 forms a capacitive voltage divider,

Of a high-frequency furnace for kw., which was extensively tested in practice, the following details are given:

Triode 1 TBW 12/25. Circuit coil 3, 3' 25 111.

Coupling coil 11 0.25;.th. Heating coil 13 0.6 111.

4 Heating coil 16 lS Lh. Circuit capacitor 4 4600 t tf. Circuit capacitor 4 4600 t tf.

The values of the circuit coil 3, 3' and of the coupling coil 11 hold for the drawn out condition of the coil core 15.

FIG. 2 shows an alternative embodiment of a device according to the invention, which can be used to advantage in a high-frequency furnace with a rated power of 6 kw. Similar elements are designed by like reference numerals. I

In this device, the circuit coil 3, 3' and the associated circuit capacitors 4, 4' are arranged in fixed relationship to one another and united to form an integral structure 23 which, to vary the coupling between the circuit coil 3, 3 and the coupling coil 11, is adapted to be displaced relatively to the stationary coupling coil 11 of the load circuit. To this end, supply leads 24, 25, 26 connecting this integral structure 23 to the valve oscillator are designed flexible.

In this embodiment, the circuit coil 3,3 is designed as a cylindrical winding and axially displaceable by displacement of the integral structure 23 relative to the insulated coupling coil 11, which encloses the circuit coil 3, 3' and consists of a cylindrically bent broad copper band. In order to achieve load matching over a maximum control range, the structure 23 is displaceable through a distance equal to the length of the circuit coil 3, 3, and the length of the coupling coil 11 is adapted to this length.

In a manner identical to that described with reference to FIG. 1, the high-frequency furnace can be used for high-frequency heating through a low-resistance load circuit and for high-frequency heating through a highresistance load circuit, and in the first case the coupling coil 11 is connected to the first heating coil 13 and the terminals 17 of the circuit coil 3, 3 are short-circuited through the interconnection 26, while in the second case the second heating coil 16 is connected in series with the circuit coil 3, 3' and the terminals 12 of the coupling coil 11 are short-circuited through the interconnection 21. The operation is entirely similar to that of the high-frequency furnace described with reference to FIG. 1

It should be remarked here that in this embodiment the reactances of the circuit capacitors 4, 4' and hence the circuit coils 3, 3' also are in the ratio 4:1.

Of, a practical embodiment of this high-frequency furnace the following of the most important details:

FIG. 3 shows a further development of the high-frequency furnace in accordance with the invention.

In this high-frequency furnace of the type shown in FIG. 1, each of the circuit coil parts 3 and 3, which are included in the anode circuit of the oscillator valve, is provided with second coupling coils 27 and 28, respectively, the output terminals of the coupling coils 27, 28 being designated by 29, 30 and 31, 32, respectively. Each of the circuit coil parts 3 and 3 together with the associated coupling coils 27, 28, respectively, can be designed as a separate coil set, and this if of particular advantage from the point of view of manufacturing technology.

Furthermore, this construction enables the coupling coils 27 and 28 to be connected, atwill, either in series or in parallel by means of interconnecting strips; thus, for parallel connection the output terminals 29, 31 and 30, 32 are connected together through interconnecting strips 33 and 34 and the output terminals 29, 30 are connected to the first heating coil 13, while for series connection the output terminals 30, 31 are connected together through an interconnecting strip 35 shown as a broken line and the output terminals 29, 32 are connected to the first heating coil 13. This step provides the important advantage that the matching control range for the low-resistance load circuit can be appreciably extended and, in particular, the transformation ratio can be increased by a factor of two by with the circuit coil means and thefirst switch shortcircuits the first heating coil to the coupling coil means.

means of series and parallel connection of the coupling coils 27, 28 so that, for example, for the low-resistance load circuit the transformation ratio can be varied between 2021 and 4:1.

Finally, it should be remarked that for special applications, for example for the simnltaneously'processes of hardening a sleeve and securing a tube therein by welding, the interconnection device of FIGS. 1, 2, and 3 enables not only the first heating coil 13 to be connected to the coupling coil but also the second heating coil 16 to be connected to the tuned anode circuit 2.

What is claimed is: p

1. A high frequency furnace for induction workpieces comprising an electron discharge device having an output electrode, oscillatory circuit means coupled to said output electrode, said oscillatory circuit means comprising a circuit coil means and capacitance means, coupling coil means in inductive coupling relationship with said circuit coil means, means to vary said inductive coupling relationship, first load circuit means having a low resistance load impedance, said first load circuit means comprising a first heating coil, second load circuit means heating of having a high-resistance load impedance, said second load a circuit means having a second heating coil, and an interconnection device having first switchingmeans to couple said first heating coil to said coupling coil means toprovide said induction heating for work-pieces associated with said first load circuit, and second switching means to short circuit said coupling coil means and couple said second heating coil to said circuit coil means for said inductive 3. A high-frequency furnace as claimed in claim 1 wherein the inductance of the. first heating coil lies between 0.02nh. and Lab. and the inductance of the'second heating coil lies between 1/.Lh. and 40/.th.

4. A heat furnace as claimed in claim 1, wherein said capacitance means is connected in parallel to said coil circuit means, said capacitance means comprising first and second series-connected capacitors having a common grounded junction, said first and second capacitors having first and second predetermined reactances, respectively, andsaid circuit coilmeans comprising a first and second circuit coils, said first and second circuit coils having a reactance ratio substantially equal to the ratio of said first and second predetermined reactances and said second switching means of said interconnection device serially connects said second heating coil between said first and second circuit coils and said first switching means serially connects said first circuit coil to said second circuit coil. u

5. A high frequency furnace as claimed in claim 4 wherein said first and second reactances are equal.

6. A high frequency furnace as claimed in claim 4 wherein said coupling coil meanscomprises a first coupling coil coupled to said first circuit coil, and second coupling coil coupled to said second circuit coil.

7. A high frequency furnace as claimed in claim 6 comprising further means to connect said first coupling coil in parallel with said second coupling coil.

8. A high frequencyfurnace as claimed in claim 6 References Cited in the file of this patent UNITED STATES PATENTS 2,321,189 Dravneek June 8, 1943 2,452,365 Frederick -f. Oct. 26, 1948 2,593,959 Anderson et al. Apr. 22, 1952 2,662,162 Blok Dec. 8, 

